Why Did They Do That?

A large part of field investigation is trying to get inside the head of whoever designed and built the structure you are looking at.* The idea is that people do things for rational** reasons so if we can figure out those reasons we have a head-start on understanding the built result. The more you understand about the tools used by the person responsible for the original design, the easier it is to understand their logic; so it’s easier to work backwards on a ten-year-old building than a hundred-year-old one.

That photo is a close up of a splice in some dunnage that will soon be removed from the roof of a building we’re working on in Greenwich Village. I don’t know when the steel was installed, but those are modern high-strength bolts, so the short answer is sometime in the last fifty years or so. Dunnage beams often have splices because they are brought up to the roof (a) in an internal elevator and the beams won’t fit in the cab or (b) using an external hoist of limited weight capacity.

The splice consists of three plates: one each at the beam’s top flange, web, and bottom flange. The bolts in the web plate are closer than the normal 3-inch spacing used in steel detailing; the bolts in the flange plates are closer than that. And I can’t figure out why. The flange-plate bolts are so close together that they interfere with one another, and could not be driven with a normal torque wrench because the wrench socket would need clearance all around the nut or bolt head. The only way these bolts could be installed is with old fashioned open-end wrenches, which was certainly more difficult for the steel workers.

I also wonder if this spacing was actually designed or was fudged by the fabricator. But that still doesn’t explain why anyone would do this. Steel plate is not expensive at all, so saving a few inches in length of the plates was meaningless. There was room along the beam length to make the plates longer. If space was really at such a premium, welding would have saved both plate length and the space occupied by the bolts.

I have often told other engineers that we’ll never know all the details of existing buildings, so we have to accept some things that we don’t understand. But this one will eat at me.


* A large part of “reverse engineering” is just this step.

** “Rational” does not mean “correct,” of course. It simply means that they followed a logical train of thought to a conclusion.

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